Method of and apparatus for vapor superheat temperature control with gas recirculation



Feb. 3, 1959 P. H. KOCH METHOD OF AND APPARATUS FOR-VAPOR SUPERHEAT TEMPERATURE CONTROL WITH GAS RECIRCULATION 4 Sheets-Sheet 1 Filed June 4, 1952 INVENTOR Paul ffjfoclz ATI'QRNEY Feb. 3, 1959 p, KOCH 2,871,833

7 METHOD OF AND APPARATUS F OR VAPOR SUPERHEAT TEMPERATURE CONTROL WITH 1 GAS RECIRCULATION Filed June 4, 1952 4 Sheets-Sheet 2 H '0 RvM Y 4 m w 5 MAN 3 Nfl R 6 w i o E a 4 M m u n H w 9 8 a A M Q P NR 2.. H 5 Y m, lql. SH B 3 .n EH. EJMW 1H 2 W 8 a I w f! 0 H 1 TJ mm v 5 J I- L in w M "w 5m 7. I I JAN 3 NJ 04 1 i. 2 7 J 0 I 2 I u m w N: v Q 9r 7. F V aw s 6 4 A W 2 \U LAIM 6 0 1%, m4 M I 8 I r .U" M m 5 n 4 v iv .w Y l l 2 A 8 n m 8 4 M 5 Afiv/ v /0 4 V I i I Feb. 3, 1959 METHOD Filed June 4, 1952 P. H. KOCH OF AND APPARATUS FOR VAPOR SUPERHEAT TEMPERATURE CONTROL WITH GAS RECIRCULATION 4 Sheets-Sheet 5 l l l 1 l I-(TTORNEY Feb. 3, 1959 Filed June 4, 1952 P. H. KOCH METHOD OF AND APPARATUS FOR VAPOR SUPERHEAT TEMPE UR ONTROL WITH GA EC ULATION VII/l 4 Sheets-Sheet 4 INVENTOR.

ATTORNEY United States Patent METHOD OF AND APPARATUS FOR VAPOR SUPERHEAT TEMPERATURE CONTROL WITH GAS RECIRCULATION Paul H. Koch, Bernardsville, N. J., assignor to The Babcock & Wilcox Company, New York, N. 'Y., a corporation of New Jersey A Application June 4, 1952, Serial No. 291,686 10 Claims. (or. 122-412 This invention relates to improvements in the construction and operation of vapor generating units having a furnace chamber wholly or mainly defined by radiantly heated vapor generating tubes and a connected convection section containing convection heated surface for superheating the vapor generated, and more particularly, to a method of and apparatus for controlling the vapor superheat temperature in a fluid fuelfired unit of the character described by the recirculation of relatively cool combustion gases from a point in the gas flow path downstream of the convection superheater to a point of introduction into the furnace and in a manner whereby a reduction in the amount of furnace wall heat absorption will be effected, to thereby correspondingly increase the heat content of the gases leaving the furnace and thus the heat available for convection superheating.

If in vapor generating units of the character described the recirculated flue gases should be intimately mixed with the combustion air supply to the fluid fuel burners, the amount of furnace wall heat absorption will theoretically be reduced by the reduction in the maximum furnace gas temperature due to the increased percentage of inert gases in the combustion zone and the consequent retardation of the rate of combustion of the burning fuel. In that event the heat content of the gas stream leaving the furnace would be correspondingly increased, making more heat available to the convection superheater than if such gas recirculation were not employed. The increase in the gas heat content is dependent upon a number of factors, including thelength and shape of the gas flow path in the furnace and its relationship to furnace wall heat absorbing surface. If this gas recirculation system should be employed in a vapor generating unit having a vertically elongated furnace chamber defined by upright fluid cooled walls, fired by one or more pulverized fuel burners located in the furnace roof at points spaced from the vertical walls and arranged to downwardly discharge a combustible mixture of pulverized fuel and air, and of such vertical extent as to normally have the combustion of the fuel substantially complete before the stream of gaseous products of combustion turn laterally at a level above the furnace chamber bottom and then upwardly into a convection gas pass positioned alongside the furnace chamber and containing a convection vapor superheater, the addition of recirculated flue gases to the combustion air prior to the burners would result in an increased mas-s and velocity of the gas stream flowing downwardly in the furnace chamber. This tends to cause the gas stream to travel farther vertically downward in the furnace chamber before its direction of travel is re versed by the furnace draft and the gas stream turned upwardly into the convection gas pass. This longer gas travel increases the radiant heat transfer to the furnace wall heat absorbing surface in the portion of the furnace chamber below the level of the gas outlet, and correspondingly reduces the heat content of the gases leaving the furnace. Thus the heat available for superheating rear side of both furnace sections.

would depend upon the net change in the heat content of the gases due to the two opposing effects above described.

In accordance with my invention vapor superheat temperatures may be more effectively controlled over a relatively wide load range in vapor generating units of the character described by introducing the relatively cool recirculated flue gases directly into the furnace chamber in a manner so as to avoid interference with the ignition and combustion of the fuel and yet so as to regulably control the amount of heat absorbed by radiation by a relatively large area of furnace wall tubes, to thereby control the heat content of the gases leaving the furnace and thus the heat content available for convection vapor superheating. More specifically, my invention involves the introduction of the recirculated flue gases in a stratum or strata flowing along one or more furnace walls in a direction substantially parallel to the direction of flow of the burning fuel stream in the furnace chamber, whereby a moving layer of low temperature gas is interposed between the high temperature burning fuel stream and the corresponding furnace wall to reduce the radiant heat absorption of the blanketed furnace wall tubes and thereby tend to increase the heat content of the gases leaving the furnace. The amount of recirculated gases and their distribution and velocity relative to the furnace walls is regulated so that the reduction in furnace wall heat absorption thereby attained will be substantially greater than the increase in furnace wall heat absorption due to any increase in length of buring fuel travel in the furnace.

The various features of novelty which characterize my invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better uderstanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which I have illustrated and described preferred embodiments of my invention.

In the drawings:

Fig. 1 is a sectional elevation of a steam boiler embodying the invention taken along the line 1--1 of Fig. 3;

Fig. 2 is a vertical section taken on the line 2--2 of Fig. 1;

Fig. 3 is a partly diagrammatic horizontal section taken on the line 3-3 of Fig. 2;

Fig. 4 is a fragmentary sectional elevation of a modified front wall construction; and

Fig. 5 is a partial horizontal section taken on the line 55 of Fig. 4.

In the drawings the invention is illustrated as embodied in a steam generating and superheating unit having a vertically elongated furnace chamber divided into two side-by-side communicating sections 10' and 10" of rectangular cross-section by a vertical division wall 12 (Fig. 2) which includes a row of steam generating tubes. The front wall 14 of both sections is defined by vapor generating tubes 16 connected at their upper and lower ends to wall headers 18 and 20. The tear or baffle wall 24 is defined by the upper parts of steam generating tubes having their upper ends connected to a steam and water drum 26, the'mid-parts 2S and of these tubes extending across a furnace gas outlet 34 through which gases flow to the convection gas pass 32 at the Below the furnace gas outlet 34 the other parts 35 of the rear wall steam generating tubes are inclined downwardly along the rear wall 36 of the furnace to the wall header 38.

The furnace outer side walls 40 and 42 are lined with steam generating wall tubes '44 and 46 respectively. The tubes 44 have their upper ends connected to a header 48 and their lower end portions 58 inclined and connected to a header 50 below one side of an ash hopper 52. The tubes 46 have their upper ends connected to the header 54 and their lower ends connected to another header 56 for the adjoining ash hopper 55. The opposite side wall 60 of the hopper 52 includes inclined parts 62 of alternate division wall tubes having their lower ends connected to the header 64. Similarly, inclined parts 66 of the remaining division wall tubes are included in the inner wall 68 of the hopper 55, with their lower ends connected to a header 70. The furnace sections are thus provided with Water cooled hoppers 52 and 55 arranged With their longitudinal axes extending from front to rear of the furnace. The steam generating surface in the hoppers and adjoining furnace walls below the level of the furnace gas outlet 34 forms a substantial portion of the radiant heat absorbing surface of the unit.

The division wall headers 71-74 together with the upper side and front wall headers are connected by tubes to the vapor and liquid drum 26, Fig. 1 showing the roof tubes 80 connecting header 18 with the drum. The upper portions 82 and 84 of the rear wall tubes 24 are directly connected to the drum'26.

Steam flows from the drum 26 through the steam supply tubes 86 to a header 88 and through the convection heated tubes of the primary superheater 90, the elements of which are disposed across the gas pass 32. The outlet endsof these coils are connected to an intermediate superheater header 92 from which the steam flows through an interstage attemperator 93, and thence to the inlet header 94 of a secondary superheater section 96. From this section the superheated steam flows to a header 98 and thence to a point of use. Each furnace section is fired by groups of multiple outlet intertube pulverized fuel burners 100 of the type disclosed, for example, in U. S. Patent No. 2,588,833, arranged to downwardly discharge combustible mixtures of pulverized fuel and air between the portions of roof tubes 80 extending across staggered rectangular burner ports 102 formed in the furnace roof. As shown in Figs. 1-3, all of the burner ports are spaced apart from one another and from the adjoining vertical furnace Walls. The roof area containing the burner ports is enclosed by a windbox 104 having filler members 106 between the front burner ports and connected to the discharge side of a suitable air heater (not shown) by a supply conduit 108. With this construction and arrangement, the combustible mixture of fuel and preheated combustion air downwardly discharging at'a relatively high velocity from each of the burner ports is rapidly ignited and combustion proceeds as the combined streams flow downwardly in the furnace sections and 10". Under the influence of the furnace draft, the burning fuel and gaseous products of combustion tend to turn towards the furnace gas outlet, 34, combustion being completed before reaching the screen formed by the tubes 28 and 30. The separated ash particles drop into the hoppers 52 and 55 and thence into the subjacent ash pits. The gaseous products of combustion flow upwardly through the convection pass 32, in successive contact with the tubes of the secondary superheater 96 and primary superheater 90. Substantially all of the steam is generated in the furnace wall tubes by radiant heat absorption from the burning fuel and gases.

In the operation of the vapor generating unit, the firing rate of the burners is adjusted in accordance with the steam demand or load, the firing rate being raised with increases in demand, and vice versa. The extent of superheating surface in superheater sections 90 and 96 is customarily set in the design of the unit to provide the desired final degree of superheat when the unit is operating at a fraction of full load, which is known as the control point. At points higher than the control point up to full loads, the firing rate required to generate the steam results in a flow of heating gases of such a quantity and temperature over the superhcaters that the absorption thereby is in excess of the heat required to attain the desired superheat temperature. The excess heat is regulably extracted from the steam being superheated by the attemperator 93 in a well known manner.

When the boiler load drops below the control point, the adjusted firing rate of the burners will result in a delivery of heating gases to the superheater sections which will be deficient in heat content to attain the desired final steam superheat temperature. This is due to the fact that the percentage of the available heat in the heating gases which is absorbed by radiation by the furnace wall progressively increases as the steam load decreases. The present invention overcomes this deficiency by providing a regulable gas recirculating system to introduce relatively cool flue gas into the furnace in an effective relationship between the stream of burning fuel and gaseous products of combustion in the furnace and one or more furnace Walls, whereby the furnace wall radiant heat absorption will be reduced and the heat content of the gases flowing to the superheater correspondingly increased. This is accomplished by effectively blanketing one or more of the furnace walls with a moving layer or stratum of relatively cool flue gases recirculated from a portion of the gas path downstream of the superheater sections and introduced into each furnace section downwardly along one or more boundary walls thereof. The recirculated gases are preferably introduced at a velocity sufficient to avoid their aspiration by the gaseous constituents of the burning fuel stream and consequent interference with ignition and burning of the fuel particles. Due to the greater density of the cool recirculated gases and gravitational effect, their entrance velocity in a downfired unit need not exceed the velocity of the burning fuel stream. In all cases however the recirculated gases while in the upper part of the furnace preferably have a velocity equal to or greater than that of the burn ing fuel stream. With this relation, a relatively thick layer of moving low temperature inert gases will flow downwardly along the selected furnace wall or walls, so that the furnace walls blanketed with this layer of relatively cool gas will receive radiant heat at a much lower rate of heat transfer from the burning fuel and gas stream.

The gas recirculation system includes horizontally extending gas discharge manifolds 110-117 having narrow bottom discharge slots 118 and supplied with withdrawn flue gases by upright ducts 120-128 (Fig. 3). Similar ducts, such as and 131 which correspond to ducts 122 and 129, respectively, Fig. 3, are provided for the other section of the furnace. The upper ends of these ducts are connected with a main 140, by the cross ducts 142, 144, and 301-303. Obliquely arranged corner partitions 306309 separate the manifolds 110113 from one another. The upright ducts are preferably provided with individual dampers with the dampers in the ducts for each manifold independently controlled, so that the flow of recirculated gases down along each furnace wall can be independently controlled. Some of these dampers 146 149 are indicated in Fig. 2, associated with the ducts 129, 122, 131, and 130, respectively.

The gas main 140, and the cross ducts connected thereto, are supplied with recirculated gases by a recirculating fan 150, the inlet of which is connected by the discharge duct 152 to the recirculated gas inlet 154 which communicates with the heating gas flue 155 at a position beyond the economizer 156. Adjustable dampers 310 are positioned in the discharge duct 152 as a means of regulating the rate of discharge of the gases to be recirculated. Alternately the rate of recirculated gas discharge may be controlled bya variable speed fan drive.

The burning fuel and air-gas mixtures move downwardly in the furnace at a substantial velocity, the burners operating in such a manner that even at the maximum velocity of introduction of fuel and air, the combustion products will not impinge upon any furnace wall to any appreciable extent.

As shown in Fig. 3, the windbox 104 for the burners for furnace section is surrounded by the manifolds 110 113, and the manifolds 114117 similarly surround the Windbox 105 for the furnace section 10'.

The gas discharge manifolds are mounted directly on the furnace roof tubes. The side manifolds 110 and 112 extend longitudinally of the tubes and the discharge slots 118 are limited in width to the inter-tube spacing by the converging side wall sections such as 141 and 143. The transverse front and rear manifolds 111 and 113 are similarly constructed and the portions of the discharge slots not obstructed by the roof tubes form successive gas discharge openings. With these arrangements the recirculated gas can be introduced into the upper portion of the furnace closely adjacent to and extending throughout the width of each upright wall. The higher specific weight of the cooler recirculated gas as compared with that of the high temperature products of combustion developed by the burners in the central portion of the furnace, and the draft from the induced draft fan effect the development and downward flow of strata of recirculated gas along the upright walls. These strata limit the radiant heat transfer from the freshly developed products of combustion to the vapor generating tubes of those walls by interposing a radiation intercepting gaseous layer between the heat radiation source and the furnace wall tubes. The presence of CO and water vapor in the flue gas enhances this effect.

Through the adjustment of dampers in the ducts 120- 133, recirculated gas can be introduced along a single wall or simultaneously along a plurality of walls at a controlled rate. Through the single adjustment of the dampers 319 associated with the discharge of the recirculating fan, 'or by the adjustment of the speed of the fan, regulation of the rate of gas recirculation along one or more Walls can be readily coordinated with indicated superheat adjustment requirements. It has been found that the flow of recirculated gases down the front walls 14 will provide a greater reduction in the furnace heat absorption than a similar flow of gases down the rear wall 24, primarily due to the greater extent of the front wall and its location opposite the direction of turning of the gas stream into the convection pass.

Similar adjustment of the speed of the recirculating fan and adjustment of the dampers 320 of the modified arrangement shown in Figs. 4 and 5, provide for the regulation of rate of gas flow in the stratum developed along the front wall 1 1a, which corresponds to the wall 14 of the Fig. 1 unit. The dampers 320 are distributed throughout the length of a recirculated gas inlet 322 which leads from the recirculated gas duct 324 to the combustion chamber 10a, the recirculated gas passing between the wall tubes 16a and then into the space 334 formed by bending alternate tubes 16b and 160 out of their wall tube alignment, as clearly indicated in Fig. 4. The recirculated gases passing between tubes 16a are deflected downwardly by a baflie 326 formed by parts of the tubes 16b and 160 and flat metallic studs 328 secured to those tubes. This baflle also includes refractory material 330 disposed on the sides of the studs facing the incoming recirculated gases. Similar studs 332 are secured to the tubes 16a below the position at which the recirculated gases pass into the gas turning space 334. With this arrangement, the recirculated gases are caused to, proceed downwardly along the wall 14a to develop a stratum immediately adjacent the interior surface of that wall. The roof tubes 80a and the side wall tubes 44a of the embodiment of Figs. 4 and 5 correspond to the tubes 80 and 44 of the Fig. 1 unit.

Figs. 4' and 5 indicate one of the side walls 40a, and they also show the wall tubes along the front wall 14a and roof tubes 80a to be connected to the same header 6 18a," which'corresponds to the header 18 in the Fig. 1 unit.

As the load on the unit is decreased below the control point, increased recirculated gas flow is required for the purpose of reducing heat transfer to the furnace walls, permitting sufiicient heat to be carried out of the furnace for convection superheating. The rate of gas recirculation through the lower load range will thus be inversely related to the rate of steam generation.

The continuous introduction of recirculated gas peripherally of the furnace in the manner described has the effect of crowding the freshly developed products of combustion into a central downflowing stream of smaller transverse section so that the central hot gas portion will be at a greater distance from the furnace walls and move at a higher velocity downward through the furnace than if the described introduction of recirculated gas was not practiced.

Variation in the fan speed or other control of the gas flow to the ducts of the recirculated gas system may be automatically effected from pertinent variables such as superheat temperature, steam flow, air flow, etc. by an appropriate control system, such system operating reversely as the load decreases from maximum load to the lowest operating load. The above described arrangement of the recirculated gas system, and its control is such that, at low load, a maximum amount of wall area of the furnace walls may be insulated by relatively high velocity strata of returned gases flowing along the walls and between the latter and the downflowing burning fuel stream. Different furnace walls may be selectively relieved of their insulating gas strata, in a manner demanded by changing load conditions. For example, it may be that the stratum of insulating recirculated gases along the rear wall 24 should be eliminated as the first step in the coordination of increasing load and the inherent tendency toward superheat change from the desired value. This can be done by closing the dampers in the upright ducts and 121, and the corresponding ducts for the other half of the furnace. If the next succeeding step required by a further increase in load is the elimination of the recirculated gas strata for one or more of the side wall surfaces and division wall surfaces, this may be accomplished by a gradual or simultaneous closing of the dampers in the corresponding ducts such as 126, 129, 122 and 123, and other corresponding ducts for the other half of the furnace. It is contemplated that there should be the minimum flow of recirculated furnace gases at a selected high load, and this may be provided for by the minimum rate of operation of the fan and a minimum how of recirculated gases through the ducts 120 and 121, and the corresponding ducts for the other section of the furnace, supplying recirculated gas to the discharge manifolds for the rear wall 24.

With only the front furnace wall and the side wall surfaces and division wall surfaces blanketed by downwardly moving strata of recirculated gas, the major portion of the periphery of each stream of high temperature gases is embraced by the strata of recirculated gases. The stream of high temperature gases thus tends to be crowded into a smaller portion of the furnace crosssection increasing the velocity of the higher temperature gases, and such crowding is at a maximum when all of the furnace wall surfaces are blanketed by the contigous moving strata of recirculated gases. Such crowding of the newly developed high temperature products of combustion to a stream of smaller cross-sectional flow area reduces the transfer of heat to the furnace wall tubes. This reduction of heat transfer is. a result of the reduction of the radiating peripheral surface of the higher temperature gas stream. As the radiating periphery of the hot gas stream is reduced, there will be a reduction in heat transfer from the main combustion zone to the surrounding low temperature gas. stream, and finally to 7. thefurnace walls. As the central hot gas stream originating from the burning fuel is the main source of heat at a temperature level substantially above that of the heat receiving wall tubes, a considerable reduction in radiant heat transmission to the walls will occur when the high temperature central stream is of smaller perimeter. A reduction in heat absorbed by the upright fluid cooled furnace walls results in a greater heat content in the gases leaving the furnace, so that the desired increased convection heating of the superheater tubes is effected.

It is to be appreciated that this invention is especially useful when incorporated in a steam generating and superheating installation which has substantially all of its steam generating surface in the form of tubes lining the furnace walls. Such tubes receive substantially all of their heat by radiation from a high temperature combustion zone within the furnace. Such installations have sufficient steam generating surfaces in the furnace walls and in advance of the convection superheater to absorb a relatively large percentage of the heat released in the furnace. I

This invention is particularly concerned with such an installation provided with downfiring burners and a gas outlet from the lower part of the furnace and having a stratum or strata of recirculated combustion gases projected downwardly, i. e. in the same direction as the direction of flow of fuel and air and in a flow path between the burning fuel stream and an adjacent fluid cooled furnace wall or walls. With such parallel downward flows of the two streams, the gravitational effect tends to maintain their separation over a substantial part of theifurnace height. By the regulation of the amount of recirculated furnace gases in response to load changes, the normal tendency of a steam generating unit of this type for the superheat temperature to decrease with decreases in load, is overcome and the superheat temperature maintained at a desired value throughout a wide load range.

Modified recirculated gas inlet shown in Figs. 4 and 5 may be positioned closely adjacent the burner roof as shown, or positioned at a lower elevation spaced from the roof, the gas introduced being effective in the limitation of heat absorption of the subjacent wall area.

When the recirculated gases are introduced in the manner above described, and at a downward velocity approximating or greater than the downward velocity of the gases developed by the adjacent fuel burners, the velocity of the whole gas mass is greater than with no gas recirculation. average gas flow path which is longer than the gas flow path without gas recirculation, as the gas stream tends to extend into the furnace section below the gas outlet 34.

This effect is enhanced when recirculated gas is dis' charged down the rear wall 24, as such gas flow tends to force the main gas stream away from the outlet 34. This greater length gas flow path results in an increased radiant heat absorption by the lower portion of the furnace wall tubes and the hopper wall tubes, and this increased heat absorption in the furnace tends to offset the reduction in heat absorption in the upper portion of the furnace resulting from the blanketing effect of the recirculated low temperature gas on the upper wall portions. Tests have shown that under such conditions the gas temperature at the outlet 34 is progressively lowered as the amount of recirculated gas is increased. While the gas temperature entering the superheater at a given load will thus be lower than without recirculation, the increased gas mass provides an increased heat content of the gases available for superheating.

With further reference to the method of control of superheat in a vapor generating and superheating unit involving a radiantly heated vapor generating section and a convection heated vapor superheater receiving gases from the furnace of the vapor generating section, this This greater velocity results in an invention contemplates methods of operation effected by the installation described and claimed herein, in combination with a control system of the type disclosed in the drawings of the common assignees pending patent application Serial No. 199,406, filed by Charles S. Smith on December 6, 1950, and entitled Method of Steam Generation and Superheat Control by Gas Recirculation and Attemperation, and Apparatus T herefor. Pertinent methods of operation can be effected by the control system disclosed by this co-pending application (199,406) and described in detail therein with reference to Fig. 9 of the drawings in that application when used in conjunction with the subject matter disclosed herein for the regulation of recirculated gas flow to maintain a predetermined superheat temperature.

Thus according to my invention steam superheat temperature can be controlled over a relatively wide range below the control point by the withdrawal of relatively cool flue gas and its regulated introduction into the furnace in a manner which will cause a reduction in furnace radiant heat absorption at any given load in this range, but without such interference with the combustion process as would cause any reduction in the maximum combustion zone temperature. While the contemplated gas recirculating operation may result in a very substantial reduction in that temperature, the lowered total furnace heat absorption increases the heat content of the gases available for convection superheating and this in conjunction with the increased amount of gas flow over the convection superheater results in an increased steam superheating effect.

While in accordance with the provisions of the statutes I have illustrated and described herein the best forms of the invention now known to me, those skilled in the art will understand that changes may be made in the method and apparatus disclosed without departing from the spirit of the invention covered by my claims, and that certain features of my invention may sometimes be used to advantage without a corresponding use of other features.

What is claimed is:

1. In a method of maintaining a predetermined vapor superheat temperature over a Wide load range of operation of a vapor generating and superheating unit, burning fuel in suspension in a vertically elongated flame path by downwardly firing a combustion zone with a fluid fuel and combustion air, generating vapor by radiant transmission of heat from burning fuel to enclosed streams of vaporizable liquid in a zone extending in the direction of firing, superheating the generated vapor by the convection absorption of heat from the combustion gases beyond the combustion zone, and maintaining a predetermined vapor superheat temperature over a wide load range of vapor generation by withdrawing partially cooled combustion gases from a position beyond the superheating zone and directing the withdrawn gases separately from and in spaced relation to the fuel and combustion air introduction and as a stratum contiguously to the zone of heat absorption for vapor generation and interposed with reference to the zone of heat absorption for vapor generation and the stream of burning fuel, the introduction of the stratum of recirculated gases, from its inception, being downwardly and substantially parallel to the direction of the burning fuel stream in the combustion zone from a position immediately adjacent the point of introduction of the recirculated gases.

2. In a method of maintaining a predetermined vapor superheat temperature over a wide load range of operation of a vapor generating and superheating unit, burning fuel in suspension in a vertically elongated flame path by downwardly firing a combustion zone with a fluid fuel and combustion air, generating vapor by radiant transmission of heat from burning fuel to enclosed streams of vaporizable liquid in a zone extending in the direction of firing, superheating the generated vapor by the convection absorption of heat from the combustion gases beyond the combustion zone, and maintaining a predetermined vapor superheat temperature over a wide load range of vapor generation by withdrawing partially cooled combustion gases from a position beyond the superheating zone and directing the withdrawn gases separately from and in spaced relation to the fuel and combustion air introduction and as a stratum contiguous to the zone of heat absorption for vapor generation and interposed with reference to the zone of heat absorption for vapor generation and the stream of burning fuel at a velocity at least as great as the velocity of the burning fuel stream into the combustion zone; the introduction of the stratum of recirculated gases, from its inception, being down wardly and substantially parallel to the direction of the burning fuel stream in the combustion zone.

3. The method of generating and superheating vapor over a range of operating loads which comprises introducing a fluid fuel and combustion air and burning the fuel in suspension in a furnace chamber in a plurality of parallel streams spaced from the boundary surfaces of the furnace chamber and discharging the heating gases produced through a gas outlet at a location remote from the point of fuel introduction, generating vapor by the radiant transmission of heat from the burning fuel to en closed streams of vaporizable liquid flowing along a boundary surface of the furnace chamber, superheating the generated vapor by the convection absorption of heat from the heating gases discharged from the gas outlet, increasing the convection superheating effect at fractional operating loads by withdrawing partially cooled heating gases from a location downstream of the superheating zone and introducing the withdrawn heating gases at a position adjacent to but spaced from the point of introduction of the fuel and combustion air in a stream directed along the boundary surface of the furnace chamber between the streams of burning fuel and enclosed streams of radiantly hcatcd vaporizable fluid in a direction substantially parallel to the direction of the burning fuel streams to form a moving stratum of recirculated heating gas at the furnace chamber side of the enclosed streams of vaporizable liquid, and increasing the amount of recirculated heating gas so introduced as the operating load decreases.

4. The method of generating and superheating vapor over a range of operating loads which comprises introducing a fluid fuel and combustion air and burning the fuel in suspension in a furnace chamber in a plurality of parallel streams spaced from the boundary surfaces of the furnace chamber and discharging the heating gases produced through a gas outlet at a location remote from the point of fuel introduction, generating vapor by the radiant transmission of heat from the burning fuel to enclosed streams of vaporizable liquid flowing along a boundary surface of the furnace chamber, superheating the generated vapor by the convection absorption of heat from the heating gases discharged from the gas outlet, increasing the convection superheating effect at fractional operating loads by withdrawing partially cooled heating gases from a location downstream of the superheating zone and introducing the withdrawn heating gases at a position adjacent to but spaced from the point of introduction of the fuel and combustion air in a stream directed along a boundary surface of the furnace chamber between the streams of burning fuel and enclosed streams of radiantly heated vaporizable liquid in a direction substantially parallel to the direction of the burning fuel streams and at a velocity at least as great as the velocity of the burning fuel streams, and increasing the amount of recirculated heating gas so introduced as the operating load decreases.

5. The method of generating and superheating vapor over a range of operating loads which comprises introducing a fluid fuel and combustion air and burning the fuel in suspension in a furnace chamber in a plurality of parallel streams spaced from the boundary surfaces of the furnace chamber and dischafging the heating gases produced through a gas outlet at a location remote from the point of fuel introduction, generating vapor by the radiant transmission of heat from the burning fuel to enclosed streams of vaporizable liquid flowing along a boundary surface of the furnace chamber, superheating the generated vapor by the convection absorption of heat from the heating gases discharged from said gas outlet, increasing the convection superheating effect at fractional operating loads by withdrawing partially cooled heating gases from a location downstream of the superheating zone and introducing the withdrawn heating gases at a position adjacent to but spaced from the point of introduction of the fuel and combustion air in a plurality of streams directed'along and about the periphery of the boundary surface of the furnace chamber between the streams of burning fuel and enclosed streams of radiantly heated vaporizable liquid in a direction substantially parallel to the direction of the burning fuel streams, increasing the amount of recirculated heating gas sointroduced as the operating load decreases, and varying the number of recirculated heating gas streams so introduced relative to the boundary periphery of the furnace chamber.

6. The method of generating and superheating vapor over a range of operating loads which comprises introducing a fluid fuel and combustion air and burning the fuel in suspension in a vertically elongated flame path in a furnace chamber in a plurality of parallel downwardly directed streams spaced from the boundary surfaces of the furnace chamber and discharging the heating gases produced through a gas outlet at a location below the combustion zone, generating vapor by the radiant transmission of heat from the burning fuel to enclosed streams of vaporizable liquid flowing along a vertical boundary surface of thefurnace chamber, superheating the generated vapor by the convection absorption of heat from the heating gases discharged from said gas outlet, increasing the convection superheating effect at fractional operating loads by withdrawing partially cooled heating gases from a location downstream of the superheating zone and introducing the withdrawn heating gases at a position adjacent to but spaced from the point of introduction of the fuel and combustion air in a stream directed downwardly along a boundary surface of the furnace chamber between the streams of burning fuel and enclosed streams of radiantly heated vaporizable liquid in a direction substantially parallel to the direction of the burning fuel streams to form a moving stratum of recirculated heating gas at the furnace chamber side of the enclosed streams of vaporizable liquid, and increasing the amount of recirculated heating gas so introduced as the operating load decreases.

7. A vapor generating and superheating unit comprising front, rear and side vertical walls defining a vertically elongated furnace chamber of rectangular horizontal cross-section having a roof at its upper end and a heating gas outlet in the lower part of said rear wall, vapor generating tubes lining said vertical walls substantially throughout their vertical extent, a convection heating gas pass at one side of said furnace chamber arranged to receive heating gases from said gas outlet, a bank of convection heated vapor superheating tubes in said gas pass for superheating the vapor generated, fuel burning means arranged to discharge a plurality of parallel streams of fiuid fuel and combustion air downwardly through said roof and in spaced relation with said vertical walls including a plurality of spaced fuel and air inlet ports in said roof having a total horizontal cross-sectional area only a minor fraction of the horizontal crosssectional area of said furnace chamber, means forming a recirculated gas inlet port at the upper end of said furnace chamber extending parallel to said front and rear walls and spaced from said fuel burning means, and means for withdrawing heating gases from a position downstream of said vapor superheating tubes and introducing the with- 1 1 drawn gases into the upper end of said furnace chamber in a stream directed through said gas inlet port and downwardly along the adjacent vertical wall.

8. A vapor generating and superheating unit comprising front, rear and side vertical walls defining a vertically elongated furnace chamber of rectangular horizontal cross-section having a roof at its upper end and a heating gas outlet in the lower part of said rear wall, vapor gen- 'erating tubes lining said vertical walls substantially throughout their vertical extent, a convection heating gas pass at one side of said furnace chamber arranged to receive heating gases from said gas outlet, a bank of convection heated vapor superheating tubes in said gas pass for superheating the vapor generated, fuel burning means arranged to discharge a plurality of parallel streams of fluid fuel and combustion air downwardly through said roof and in spaced relation with said vertical walls including a plurality of spaced fuel and air inlet ports in said roof having a total horizontal cross-sectional area only a minor fraction of the horizontal cross-sectional area of said furnace chamber, means forming a recirculated gas inlet port in said roof adjacent and extending parallel to said front wall and spaced from said fuel burning means, and means for withdrawing heating gases from a position downstream of said vapor superheating tubes and introducing the withdrawn gases into the upper end of said furnace chamber in a stream directed downwardly through said gas inlet port along said front wall.

9. A vapor generating and superheating unit comprising front, rear and side vertical walls defining a vertically elongated furnace chamber of rectangular horizontal cross-section having a roof at its upper end and a heating gas outlet in said rear wall adjacent to and above its lower end, vapor generating tubes lining said vertical walls substantially throughout their vertical extent, a convection heating gas pass at one side of said furnace cham ber arranged to receive heating gases from said gas outlet, a bank of convection heated vapor superheating tubes in said gas pass for superheating the vapor generated, fuel burning means arranged to discharge a plurality of parallel streams of fluid fuel and combustion air downwardly through said roof and in spaced relation with said vertical walls including a plurality of spaced fuel and air inlet ports in said roof having a total horizontal cross-sectional area only a minor fraction of the horizontal cross-sectional area of said furnace chamber, means forming a recirculated gas inlet port in said roof adjacent and extending parallel to said rear wall and spaced from said fuel burning means, and means for withdrawing heating ases from a position downstream of said vapor superheating tubes and introducing the withdrawn gases into the upper end of said furnace chamber in a stream directed downwardly through said gas inlet port along said rear wall.

10. A vapor generating and superheating unit comprising front, rear and side vertical walls defining a vertically elongated furnace chamber of rectangular horizontal cross-section having a roof at its upper end and a heating gas outlet in the lower part of said rear wall, vapor generating tubes lining said vertical walls substantially throughout their vertical extent, a convection heating gas pass at one side of said furnace chamber arranged to receive heating gases from said gas outlet, a bank of convection heated vapor superheating tubes in said gas pass for superheating the vapor generated, fuel burning means arranged to discharge a plurality of parallel streams of fluid fuel and combustion air downwardly through said roof and in spaced relation with said vertical walls including a plurality of spaced fuel and air inlet ports in said roof having a total horizontal cross-sectional area only a minor fraction of the horizontal cross-sectional area of said furnace chamber, means forming recirculated gas inlet ports in said roof adjacent and extending parallel to said front and rear walls and spaced from said fuel burning means, and means for withdrawing heating gases from a position downstream of said vapor superheating tubes and introducing the withdrawn gases into the upper end of said furnace chamber in streams directed downwardly through said gas inlet ports along said front and rear walls, and damper means for independently controlling the recirculated gases introduced at each gas inlet port.

References Cited in the file of this patent UNITED STATES PATENTS 1,739,594 Jackson Dec. 17, 1929 1,791,955 Cannon Feb. 10, 1931 1,860,366 Lucke May 31, 1932 1,933,020 Learnon Oct. 31, 1933 2,224,544 Keller Dec. 10, 1940 2,229,643 De Baufre Jan. 28, 1941 2,537,042 Fink Jan. 9, 1951 FOREIGN PATENTS 504,114 Great Britain Apr. 14, 1939 516,070 Great Britain Dec. 21, 1939 523,870 Great Britain Dec. 21, 1939 

